Sounds like your card might be corrupt.
Try this photo recovery software. Do a deep scan.
Download the program http://www.recuva.com? You must not take anymore pictures on your card. You have to use a card reader either on your computer or an external USB card reader.Put the card in the card reader, open the recuva program, the welcome to the recuva wizard will open, click on next. next box, choose 'pictures', click on next, next box choose 'on my media card' click next. See the words 'enable deep scan' click on start. Wait, for it to do it's magic.
You need to use a card reader either one already in your computer (some have one, some don't). Or buy an external USB card reader. They are around 10.00 and can be found at walmart, bestbuy, frys, probably target.
The problem with built in card readers is that they will usually only read up to a 2G card.
I was not able to recover photos by hooking the camera up to the pc with the usb cord, it just won't work.
This is the important part. Choose the pics you want to recover, then choose recover, a box comes up asking where you want to save to. Choose documents/my pictures or wherever you want to save your pictures too. The pictures ONLY save to your pc and not the card.
Format your card in the camera after you have retrieved your photos and backed them up to an external source.